Electrochemical determination of trace amounts of environmentally important dyes

A number of dyes exhibit genotoxic or ecotoxic properties leading to the need for sensitive and selective methods for their determination. Because of the easy reducibility of dyes, modern polarographic and voltammetric methods (differential pulse polarography on classical dropping mercury electrode, differential pulse voltammetry on hanging mercury drop electrode or adsorptive stripping voltammetry) are suitable for the determination of trace amounts of these substances in the general environment in the vicinity of production plants. The scope and limitations of these methods is reviewed and optimum conditions for recently developed methods are summarized. It is shown that the sensitivity of newly developed polarographic and voltammetric methods is sufficient even for the most demanding applications and their selectivity can be increased by their combination with preliminary separation using thin layer chromatography or liquid extraction.     

Ab initio cluster model approach to the chemisorption of NH3 on Pt(111)

The chemisorption of NH₃ on Pt(111) has been studied using several Pt₁₀ clusters that model different adsorption sites of the Pt(111) surface. Ab initio methods have been used to obtain a theoretical estimate of several spectroscopic features that can be directly compared to experiment. The comparison includes the variation of the difference between the 3a₁ and 1e levels, the vibrational frequency shifts and the order of stability on different surface sites. Chemisorption at the on-top site is predicted to be favoured, the calculated interaction energy appears to be quite close to the experimental estimate, and it is suggested that NH₃ chemisorbs molecularly, in an N-down orientation, with an equilibrium geometry representing a small distortion from the gas-phase molecular geometry and no azimuthal preference, in good agreement with ESDIAD experiments. Constrained space orbital variation (CSOV) analysis of the interaction has also been performed using a Hartree–Fock wave function. This analysis shows that the leading bonding mechanisms are substrate polarisation and charge transfer from ammonia to the surface.     

Transition between the tent map and the Bernoulli shift

We introduce a one-dimensional map which can be considered as a transition from the tent map to the Bernoulli shift map. This map is a simple example of a system with two different attractors, for certain values of the parameter, with well defined basins of attraction.     

Low frequency oscillations in semi-insulating GaAs: a nonlinear analysis

We have observed low frequency current oscillations in a semi-insulating GaAs sample grown by low temperature molecular beam epitaxy. For this, an experimental setup proper to measure high impedance samples with small external noise was developed. Spontaneous oscillations in the current were observed for some bias conditions. Although measurements were carried out from room temperature down to liquid helium, the dynamical analysis was carried out around 200 K where the signal to noise ratio was fairly favorable. To increase the data quality we have also used a noise reduction algorithm suitably developed for nonlinear systems. We observed attractors having low embedding dimension, limit cycle bifurcations, and chaotic behavior characteristic of nonlinear dynamical processes in route to chaos. Attractor reconstruction, Poincare sections, Lyapunov exponents, and correlation dimension were also analyzed.     

Evidence for a diffuse transition in lead chloride

The Raman spectra and the electrical conductivity of lead chloride single crystals at various temperatures, up to the melting point, have been investigated.From the results of both techniques we have found evidence of the beginnings of a diffuse transition to a state of high conductivity at temperatures close to the melting point of PbCl₂.     

Effect of ellipsodial fermi surfaces and relaxation processes on the indirect exchange in the Bloembergen-Rowland approximation

We investigate the effect of relaxation processes (mean free path) and ellipsodial Fermi surfaces on the indirect exchange interaction in the Bloembergen-Rowland approximation and obtain a simple analytic expression which oscillates with mean free path and emphasizes the relative importance of this parameter on the oscillatory behaviour of the indirect exchange.     

On the origin of the exchange interaction in EuO and EuS

The indirect magnetic interaction in semiconductors is used to evaluate the exchange parameters for EuO and EuS respectively which yields J_eff(EuO)?(0.06 ± 0.02) eV and J_eff(EuS)?(0.08 ± 0.04) eV.     

The role of polymer spacers in specific adhesion

We study the role of flexible spacers in specific adhesion from the point of view of polymer reaction--diffusion theory. By assuming that the interactions between complementary adhesion moieties occur on a length scale much smaller than the size of the polymer spacer, we describe in detail binding and rupture between two opposing surfaces. Predictions are given for the physical properties of interest such as the time evolution of bond density and the ranges of attraction and unbinding. We also discuss the dynamic crossover between reversible and irreversible bridging.